why is refractive index of a medium always greater than 1?
The refractive index of a medium is generally defined to be the refractive index of the medium relative to air/vacuum. Mathematically, refractive index is given as
Now, as you know that speed of light is maximum in vacuum, therefore, we always get refractive index of a medium to be greater than one.
The given diagram shows the lateral shift of light rays when it passes through thin and thick glass slabs.
The lateral shift (lateral displacement) depends on the following factors:
(1) Thickness of the glass slab.
(2) Refractive index of the glass slab and air.
(3) Angle of incidence of the ray.